8gb iPhone says storage is almost used up?

I have an iPhone 4 that I have had for a few months, it's 8gb and has been giving me trouble
Every once in a while when I try to take a picture it will pop up with a warning that says "Not enough memory to take another photo." which doesn't make sense because I have only used 6.3 gb, but the warning goes away after I turn my phone off
But just now as I was trying to get on Instagram my phone had a warning pop up that said
that my phone is almost at full capacity, but shows that I still am only using 6.3gb
I'm not sure why it's coming up with these warnings,
but I think it's because of some of my apps, is there a way to clean these apps up?
Or is it a completely different reason?

Even though it says it is 8 GB doesn't mean you have 8 GB for you to use. System files and other data have to use the 8 GB's as well. There's no way to delete these because the phone needs them to run. I would suggest getting rid of any apps you don't use that often or don't need and maybe getting rid of other files like music, videos and photos that you don't want.

Ok look even though it says its an 8 gb it actually uses up apps and others things that come included in the phone so it's not really 8 gb but less that's probably why that happens to you